Electro-photographic means and method



FIG. 4

July 29, 1958' H. KALLMAN' 2,845,343

' ELECTRO-PHOTOGRAPHIC MEANS AND METHOD Filed Jan. 4. 1952 FIG. I FIG. 2

FIG. 3

Pie. 6

IN V EN TOR.

HAR'TMUT KALLMANN United States Patent ELECTRO-PHOTOGRAPHIC MEANS AND METHOD Hartmut Kallman, Queens, N. Y., assignor to the United States of America as represented by the Secretary of the Army Application January 4, 1952, Serial No. 265,023

5 Claims. (Cl. 96--1) (Granted under Title 35, U. S. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

This invention relates to a method and to the means employed for practicing the art of electro-photography and relates particularly to a specific class of photo-sensitive elements and the treatment thereof thruout the process.

The basic principles of electro-photography are known and have been developed to a limited extent.

The present invention provides a sensitive element having properties which differ from the elements used heretofore. The element used in the present invention has a coating having a high dark resistivity and also possesses the ability to respond in a specific manner to the irradiation thereof with visible light. This special property causes the element to assume a pattern of conductivity varying in proportion to the intensity of the light directed thereon. When an image formed by a lens or other means is directed upon the sensitive element a latent image of conductivity is formed therein. At the end of the exposure period the degree of conductivity of all the areas of the film then remain unchanged so long as the element is shielded from light. The pattern of conductivity thereon corresponds in position and degree to the light and dark areas of the subject.

The latent image is then developed by charging the sensitive element with electricity by subjecting it to an electric field. An electro static image is thus created corresponding to the electrical conductivity image.

The charged element is then presented to a quantity of finely divided material such as chalk which preferably is suspended in air. This fine powder is attracted to the element and a pattern is formed having the same conformation and tonal scale as the subject since the powder is attracted in amounts proportional to the energy of the charge upon each area of the element. highly charged areas accumulate a relatively dense layer of powder which forms the shadow portions of the picture while the lesser charged areas make up the half tones and highlights of the picture. This powder image may be afiixed directly to the element itself or preferably it is offset to another surface and afiixed thereto in any suitable way to provide a permanent print.

Prior to the present invention the means for creating an electro static image upon a photo sensitive element necessitated an arrangement whereby the sensitive ele- The present invention permits the use of a conventional camera of any desired type thus entirely overcoming the impractical aspects above related in connection with prior methods of practicing electro-photography.

It is a primary object of the invention to provide a photographic system which is capable of producing reproductions of a subject with great rapidity after an exposure has been made.

A further object of the invention is to provide an electro-photographic process wherein the picture taking camera or other exposure means is or may be a separate and readily portable unit of conventional construction.

A further object of the invention is to provide a photographic process in which no solutions or washing techniques are required.

A still further object of the invention is to provide an electrophotographic process wherein a lasting latent image is initially created independently of a subsequent electric charging step.

A still further object of the invention is to provide a photo sensitive element which can be resensitized and reused many times.

Thus the more I ment is electrically charged duringthe exposure which Other objects and features of the invention will more fully appear from the following description and will be particularly pointed out in the claims.

To provide a better understanding of the invention a particular embodiment thereof will be described and illustrated in the drawings in which- Fig. l is a cross-section thruthe picture recording element embodying the invention.

Fig. 2 is a cross-sectional view of an apparatus for irradiating the recording element.

Fig. 3 is a cross-sectional view of a conventional camera showing diagrammatically the arrangement of subject and the picture taking element.

Fig. 4 is a diagrammatic view illustrating the means for charging the picture taking element.

Fig. 5 is a diagrammatic view illustrating a device for developing the picture upon the picture taking element.

Fig. 6 is a side view showing the relationship of the elements necessary for ofiset printing of the picture from the sensitive element to a permanent support.

Fig. 7 is an illustration of the final print reproducing the subject.

As in conventional photographic processes the invention provides a light sensitive element 10 upon which a record of the subject is made. The element 10 may be permanently fiat or it may be a flexible sheet or strip depending upon the type of camera in which it is used. The element 10 is composed of a support 11 of electrically conductive material such as copper, brass or the like upon which a closely adherent thin layer 12 of photo sensitive material is applied.

l have found that phosphors such as zinc cadmium sulfide phosphor or zinc cadmium selenide phosphor have photo conductive characteristics which are employed in the present invention to produce extremely beneficial results in the art of photography.

The phosphors can be treated to provide'a high degree of dark resistivity and when exposed to light their electrical resistivity is temporarily reduced substantially in proportion to the energy of the light reaching the material. The phosphors may be used in finely powdered form or they may be used in a natural crystalline form.

This property of the phosphors is applied to the practice of the invention by coating the support 11 with a thin layer of activated material. Activation may be achieved in a known manner as by retaining controlled amounts of impurities such as copper and manganese in the material. The coating may be done in any suitable manner as by mixing the powder or crystals with a small amount of binder such as Duco and spraying the mixture upon the support to provide a thin layer in which the phosphor is evenly dispersed. Another means of applying the coating would be to mix the powder or crystals in a suitable liquid suspension and applying the liquid by spraying or any conventional manner such as painting upon the surface of the support.

A flexible support can be used in place of the relatively rigid plate 11 shown in the drawings in which case the support may be thin metal sheeting or a flexible plastic sheet or strip upon which a thin metallic layer may be applied as by sputtering thereon or vaporization in a vacuum chamber. In this latter case the layer 12 is applied directly to the metallic coating.

To complete the element it is necessary only to place it in an irradiation chamber 13 such as that shown in Fig. 2 containing a source of infrared radiation 14. This treatment causes the layer to become highly nonconductive electrically and is then ready to be used in the camera. At this time the element 16 must be protected from visible light and may be kept in a light tight holder 15 or any light tight container.

To produce a picture the holder 15 is placed in a camera 16 in position to hold the element 10 at the focal plane thereof and its dark slide 17 is removed. An exposure of the subject 18 is then made thru the camera lens in the conventional manner. The image thus impressed upon the element 10 induces different degrees of conductivity over the areas of the layer 12 in a pattern representing the different degrees of light intensity as they exist upon the subject. Thus the picture taken is depicted upon the plate as a latent conductivity image. This conductivity pattern will persist within the layer for a relatively long period of time which permits development of the picture at any convenient time subsequent thereto without degrading the accuracy and quality of the original image.

The above qualities exhibited by the layer may be termed the quality of photo conductivity in a phosphor and such terminology will be used subsequently to aid in presenting the invention.

The subsequent processing may be done in an adequately equipped laboratory or dark room under closely controlled conditions to produce a high quality picture. However, the processing is no more critical than any existing photographic process.

After taking the picture, the dark slide 17 is replaced in the holder 18 and the exposed element is transferred to a light tight housing containing the picture developing means. Within the housing which is not shown is an electric charging apparatus which may be of any suitable construction and may consist of a source of electric energy 19 indicated diagrammatically in Fig. 4 one terminal of which is connected to an electrode 2% which desirably has at least the same area as the element 10 and which is placed closely adjacent and in parallel relation thereto. The other terminal of the power source is connected to the support 11 of the plate 10 and the switch 21 is closed for a predetermined time during which the layer 12 is subjected to a strong controlled electric field. The length of time and the intensity of the field is chosen to produce the best results for a given type of picture. During the charging period the latent conductivity pattern created in the layer by exposure in the camera causes the layer to assume an electro static charge pattern corresponding to the subject. At those areas where the conductivity is greatest the charge will leak off to the metallic support 11 and at areas of low conductivity the charge will not leak off to as great an extent. Therefore, at the low conductivity areas a relatively high energy charge is formed. All areas of the picture will assume a charge substantially in proportion to their respective conductivity. When the charging power is cut off the charge at each area will become trapped or localized therein for a reasonably long period of time. However, since the apparatus for developing the picture can readily be sit- 4 uated in the same inclosure with the charging apparatus or closely adjacent thereto, the charged plate desirably is immediately presented to the dusting mechanism shown diagrammatically in Fig. 5.

The dusting operation may be accomplished in any suitable manner such as that shown by way of example in the drawings in which the charged element 10 is presented to a supply of finely powdered material such as chalk. Any suitable finely divided material which is attracted by a charged element may be used. This powder may be of a preselected color to impart a desired effect to the finished print. It is also within the realm of possibility to employ the present invention not only in monochrome photography but also in full color reproductions.

The powder is contained in a receptacle 22 which desirably is provided with an agitator 23 which may be rotated by hand or in any suitable manner to cause the powder to become suspended in the air above the powder mass to form a dust cloud.

The element 10 is placed over the top of the dust container with its charged surface 12 exposed to the dust cloud. The particles of dust in suspension will then follow the natural law which causes them to be attracted by the charged surface. Moreover, the number of such particles attracted to the various areas of the layer will be substantially in proportion to the intensity of the charge thereon. Thus, a picture will be formed on the element by the particles consisting of highlights, shadows, and half tones depicting those of the subject.

The dust particles which actually have now become the pigment of the printing process may be fixed directly upon the element 10 as by means of spraying with a fixative or preferably the pigment is offset upon another support such as a sheet of paper 24 or any desired material. To transfer the pigment, the surface of the paper 24 may be coated with an adhesive and the plate 10 applied thereto with pressure, thus offsetting the pigment from the member 10 to the paper which then becomes the final print 25. Obviously, a negative of this print can be made in any desired manner from which additional positive prints may be made.

An additional feature of the invention resides in the ability to control the contrast of the final reproduction. This can be done by controlling the degree of irradiation by infrared light of the element 10 before its exposure in the camera. The normal contrast for a continuous tone type of picture may be determined by experimentation and then greater degrees of contrast may be achieved by additional infrared radiation of the sensitive layer.

It should be pointed out that the sensitive layer 12 is reusable to practically an unlimited extent. When the powder image is offset to produce the final print the layer 12 may be resensitized by irradiation with infrared light after which it may be again exposed and processed.

What is claimed is:

l. A method of practicing electro-photography comprising preparing an uncharged thin layer containing a photo conductive phosphor, exposing said layer to a pattern of light to be reproduced thereby to produce a latent image of conductivity in said layer, then electrically charging said layer in darkness after said exposure, developing said image in darkness by presenting said layer closely adjacent to a quantity of freely displaceable fine particles capable of attraction to said charged layer in an amount selectively proportional to the energy in the various areas of the pattern and fixing the particles in their respective positions to produce a permanent print.

2. A method of practicing electro-photography comprising coating a support with an uncharged thin layer containing a photo conductive phosphor, exposing said layer to a pattern of light to be recorded thereby to produce a latent image of conductivity in said layer, subsequently electrically charging said layer in darkness after said exposure, developing said image in darkness by presenting the layer to a quantity of freely displaceable fine particles subject to attraction by said charged layer in an amount selectively proportional to the energy present in the various areas of the image and fixing said particles in their respective positions to produce a final picture.

3. A method of practicing electro-photography comprising preparing an uncharged photo sensitive layer capable of taking and retaining a latent image of conductivity, irradiating said layer with infrared light, exposing said layer to a pattern of visible light to be recorded thereby to produce a latent image of conductivity therein, subsequently electrically charging said layer in darkness after the exposure to visible light thereby to convert the conductivity image to an electrostatic image, developing the image on said layer in darkness by dusting it with fine particles capable of adhering to the layer in an amount proportional to the energy contained in the various areas thereof and fixing said particles in their respective positions to produce a permanent picture.

4. A method of practicing electro-photography comprising preparing an uncharged thin layer containing a photo conductive phosphor characterized in that it has a high dark resistivity and that it will selectively change its resistivity in proportion to the intensities of a light pattern impressed thereon, protecting said layer from light until is is exposed, subsequently exposing said layer to a light pattern, thereafter protecting the layer from light, electrically charging said layer while it is protected and then developing said layer.

5. A method for practicing electro-photography according to claim 4 and wherein the said sensitive layer is irradiated with infrared radiations prior to its exposure to a-visible light pattern.

References Cited in the file of this patent UNITED STATES PATENTS 2,169,840 Lewis et al Aug. 15, 1939 2,221,776 Carlson Nov. 19, 1940 2,277,013 Carlson Mar. 17, 1942 2,292,914 Wesch Aug. 11, 1942 2,297,691 Carlson Oct. 6, 1942 2,546,160 Lengyel Mar. 27, 1951 2,551,582 Carlson May 8, 1951 2,588,569 Picard Mar. 11, 1952 2,599,542 Carlson June 10, 1952 2,600,580 Sahel et al June 17, 1952 2,624,857 Mager Jan. 6, 1953 2,663,636 Middleton Dec. 22, 1953 2,692,178 Grandadam Oct. 19, 1954 FOREIGN PATENTS 188,030 Great Britain Oct. 23, 1922 271,401 Great Britain June 16, 1927 375,639 Great Britain June 30, 1932 464,112 Great Britain Apr. 12, 1937 

1. A METHOD OF PRACTICING ELECTRO-PHOTOGRAPHY COMPRISING PREPARING AN UNCHARGED THIN LAYER CONTAINING A PHOTO CONDUCTIVE PHOSPHOR, EXPOSING SAID LAYER TO A PATTERN OF LIGHT TO BE REPRODUCED THEREBY TO PRODUCE A LATENT IMAGE OF CONDUCTIVITY IN SAID LAYER, THEN ELECTRICALLY CHARGING SAID LAYER IN DARKNESS AFTER SAID EXPOSURE, DEVELOPING SAID IMAGE IN DARKNESS BY PRESENTING SAID LAYER CLOSELY ADJACENT TO A QUANTITY OF FREELY DISPLACEABLE FINE PARTICLES CAPABLE OF ATTRACTION TO SAID CHARGED LAYER IN AN AMOUNT SELECTIVELY PROPORTIONAL TO THE ENERGY IN THE VARIOUS AREAS OF THE PATTERN AND FIXING THE PARTICLES IN THEIR RESPECTIVE POSITIONS TO PRODUCE A PERMANENT PRINT. 